1. Field of the Invention
The present invention relates to a sodium ion secondary battery including a positive electrode, a negative electrode, and a non-aqueous electrolyte.
2. Description of the Background Art
Non-aqueous electrolyte secondary batteries have recently been in wide use as secondary batteries having high energy densities. The non-aqueous electrolyte secondary battery uses a non-aqueous electrolyte and carries out charge and discharge by allowing for example lithium ions to be transferred between its positive and negative electrodes.
Such a non-aqueous electrolyte secondary battery generally uses a lithiumtransitionmetal composite oxide having a layered structure such as lithium nickelate (LiNiO2) and lithium cobaltate (LiCoO2) for a positive electrode and a material capable of intercalating and deintercalating lithium such as a carbon material, a lithium metal, and a lithium alloy for a negative electrode (see for example JP 2003-151549 A).
The use of the non-aqueous electrolyte secondary battery described above results in a discharge capacity from 150 mAh/g to 180 mAh/g, a potential of about 4 V, and a theoretical capacity of about 260 mAh/g.
The non-aqueous electrolyte is produced by dissolving an electrolytic salt such as lithium tetrafluoroborate (LiBF4) and lithium hexafluorophosphate (LiPF6) in an organic solvent such as ethylene carbonate and diethyl carbonate.
In the conventional non-aqueous electrolyte secondary battery that uses the lithium ions as described above, an oxide of cobalt (Co) or nickel (Ni) is mainly used for its positive electrode and these materials are limited as natural resources.
If all the lithium ions are deintercalated from the lithium nickelate or the lithium cobaltate in the non-aqueous electrolyte secondary battery described above, the crystal structure of the lithium nickelate or the lithium cobaltate is destroyed. As a result, oxygen is released from the lithium nickelate or the lithium cobaltate, which may cause a safety concern. Therefore, the discharge capacity cannot be increased from the above-described level.
Manganese (Mn) available in abundance as a resource may be used in place of nickel or cobalt, but the capacity of the non-aqueous electrolyte secondary battery is halved in the case.
It is difficult to produce lithium manganate (LiMnO2) having a layered structure effective in improving the mobility of lithium ions using manganese. Therefore, lithium manganate (LiMn2O4) having a spinel structure is generally used. With the LiMn2O4, if all the lithium ions are deintercalated, the state of MnO2 is maintained. Since manganese is stable in a tetravalent state, it does not release oxygen and is therefore considerably safe.
The use of LiMn2O4 allows a potential of 4 V to be obtained, but a discharge capacity only from 100 mAh/g to 120 mAh/g can be obtained.
Furthermore, attempts to produce layered LiMnO2 have been made but when the potential becomes about as low as 3 V and the charge and discharge cycle is repeatedly carried out, the LiMnO2 is transformed into spinel LiMn2O4. It is believed that the layered LiMnO2 is chemically unstable because the radius of the lithium ions is small.